This disclosure relates to determining oil-based mud contamination of native formation fluids downhole.
This section is intended to introduce the reader to various aspects of art that may be related to various aspects of the present techniques, which are described and/or claimed below. This discussion is believed to be helpful in providing the reader with background information to facilitate a better understanding of the various aspects of the present disclosure. Accordingly, it should be understood that these statements are to be read in this light, and not as an admission of any kind.
Wells can be drilled into a surface location or ocean bed to access fluids, such as liquid and gaseous hydrocarbons, stored in geological formations. The formations through which the well passes can be evaluated for a variety of properties, including but not limited to the presence of hydrocarbon reservoirs in the formation. Wells may be drilled using a drill bit attached to the end of a “drill string,” which includes a drillpipe, a bottomhole assembly, and additional components that facilitate rotation of the drill bit to create a borehole. During the drilling process, drilling fluid, which may be referred to as “mud,” is pumped through the drill string to the drill bit. The drilling fluid provides lubrication and cooling to the drill bit during the drilling operation, and also evacuates any drill cuttings to the surface through an annular channel between the drill string and borehole wall. Drilling fluid that invades the surrounding formation may be referred to as “filtrate.”
It may be desirable to evaluate the geological formation through which the borehole passes for oil and gas exploration (e.g., to locate hydrocarbon-producing regions in the geological formation and/or to manage production of the hydrocarbons in these regions). Evaluation of the geological formation may include determining certain properties of the fluids stored in the subsurface formations. When a sample of the fluid in the borehole is collected for evaluation of the subsurface formation, the sample fluid may include formation fluid, filtrate, and/or drilling fluid. As used herein, “formation fluid” refers broadly to any fluid (e.g., oil and gas) naturally stored in the surrounding subsurface formation. To sample or test the fluid, a downhole acquisition tool may be moved into the wellbore to draw in the fluid.
Fluids other than native reservoir fluid (e.g., uncontaminated formation fluid) may contaminate the native reservoir fluid. Therefore, the fluid drawn from the wellbore may be a mixture of native reservoir fluid and drilling mud filtrate. Of certain concern are oil-based mud drilling fluids that may be miscible with certain native reservoir fluids (e.g., oil and gas). The miscibility of the oil-based mud and the native reservoir fluid may cause difficulties in evaluation of the native reservoir fluid for assessing the hydrocarbon regions; in particular, the region's economic value. Accordingly, the collection of the native formation fluid may involve drawing fluid into the borehole and/or the downhole acquisition tool to establish a cleanup flow and remove the mud filtrate contaminating the formation fluid.